turn on SELinux and Firewall FC5

I installed isp config as well works great. But I want to turn on my SELinux and Firewall. Is this a good idea? During the installation process (FCr_perfect_installation) I had to turn of these to programms. Did anybody turn them on after installation or should I leave them as they are?
If I should turn them on now is their any help or tutorial or howto?